This application relates to satellite navigation systems, and more particularly to determining a satellite antenna attitude with an antenna array having multiple antennas.
Antenna configurations for GPS receivers can include either a single antenna which can be controlled to point in different directions, or multiple antennas in the form of a linear or a two-dimensional array. Antenna arrays may have the advantage of having fewer, if any, moving parts. The attitude of an antenna array may be measured by determining a phase difference between the various antenna elements of radio signals received from a GPS satellite. The antenna elements are typically spaced apart at regular intervals. If the spacing or baseline between the antenna elements is less than half the radio wavelength of the signals, then the surface in angle space along which the signal must lie can be determined unambiguously. This determination can be made for more than one GPS satellite visible in the sky, with the intersection of the surfaces defining the orientation of the antenna array with respect to the datum. The accuracy of the attitude measurement increases with increasing distance between the array antenna elements (baseline) due, for example, to the improved signal-to-noise ratio and improvement in other non-ideal properties of the antenna array. When the antenna baseline is greater than half a wavelength of the GPS satellite carrier frequency, the solution becomes ambiguous, as is known in the art. Since the position of each GPS satellite is accurately known relative to the Earth""s coordinate frame, as is known in the art the GPS receiver can accurately locate the position in three-dimensional coordinate space using a single antenna, and only the attitude is determined from the carrier-phase interferometry. The ambiguities are resolved by ensuring that the order N of the ambiguity for each baseline and GPS satellite yields a solution consistent with the known positions of the GPS satellites at any instant in time.
The attitude of the antenna array relative to a signal source such as a satellite can be determined with sufficient accuracy if the phase delay between a signal incident on an antenna element of the antenna array and the receiver is substantially independent of the angle at which the signal is received. As is known in the art, however, antenna element measurements do exhibit some angle-dependent variations in phase delay. While such variations, which will also be referred to herein as phase center errors, are typically relatively small, and do not ordinarily prevent ambiguities from being resolved, they may still cause errors in the attitude determination, particularly for small baselines. One source of variation is the antenna itself. Practical antennas and antenna elements have a phase response that is a function of elevation and azimuth. For a stationary antenna or antenna array, as the satellite moves in space, the angle of incidence of the radio wave changes with respect to the antenna coordinates, introducing a phase error in the measurement which corresponds to an error in angle. Although this error has been discussed for a single satellite and a single antenna pair, it can be generalized to apply to multiple satellites and multiple antenna pairs. Depending on the orientation of the antenna array, the effects will differ in the pitch, roll and yaw axes. Since the satellites are in motion with respect to a fixed antenna, the angle-dependent error of the antenna array will result in a time-dependent error in the angle measurement and consequently also in the antenna attitude determination.
The angle measurement error is composed of at least two components: (i) short-time period fluctuations in the signal due to electrical noise, and (ii) a phase center error of the antennas as a function of angle of incidence of the radio wave received from the satellite. The first, short-time noise can be characterized as a first order Markov process (low-pass filtered white noise). This error can be reduced by averaging the measurements over a predetermined time period, for example, over several seconds. The averaging time will depend on the change of the relative angular position of the satellite with time and the antenna characteristics, since the satellite should be substantially stationary during the averaging time. The second error, due to the phase center errors can be shown experimentally to have a periodicity of the order of the sidereal day, since the pattern of satellite motions repeats. Accordingly, in order to eliminate the phase center error, the carrier-phase attitude could be measured over a sidereal day, which is not practical. The error in angle estimate therefore may not approach a theoretical antenna resolution for antenna arrays having a relatively short baseline of approximately 2-10 meters.
It would therefore be desirable to provide a system and a method for correcting the effects due to phase center errors, in particular, but not solely, of antenna arrays used in static applications, such as the determination of the orientation of a platform with respect to True North for tactical spotting scopes and/or orientation of artillery.
The invention relates to methods and systems for determining the attitude of an antenna array.
According to one aspect of the invention, a method of determining a satellite antenna attitude comprises providing a plurality of coupled antenna elements, which receive radio signals from a satellite, changing an orientation of the plurality of elements with respect to a reference plane over a predetermined angle, locally monitoring the orientation of the plurality of elements, calculating a difference between the locally monitored orientation with respect to the reference plane and the attitude determined from the received satellite radio signals as the orientation of the plurality of elements is changed; computing an average of the difference; and correcting the attitude of the plurality of elements based on the average. The average may be a time average. The orientation of the plurality of elements may be locally monitored using an encoder, a shaft encoder or a gyro. The plurality of elements may be mounted on a common platform, with the antenna elements having a common orientation with respect to one another, and the orientation of the plurality of elements with respect to a reference plane being adjusted by adjusting the orientation of the common platform. The coupled elements may include a substantially linear array or a substantially two-dimensional array.
According to another aspect of the invention, a system for determining a satellite antenna attitude comprises a plurality of coupled antenna elements receiving radio signals from a satellite, a receiver which receives antenna signals from the coupled elements and determines a phase relationship between the radio signals received by the plurality of coupled elements, a local attitude monitoring device which monitors an orientation of the plurality of coupled elements with respect to a reference plane, and a processing device which receives from the local attitude monitoring device a first signal corresponding to the orientation of the plurality of elements with respect to the reference plane and which receives from the receiver a second signal corresponding to the phase relationship, wherein a direction of the plurality of elements with respect to a position of the satellite is changed over a predetermined angle and a correction value of the attitude of the plurality of elements is determined from an averaged difference between the first and second signal. The averaged difference may be a time-averaged difference. A drive means coupled to at least a portion of the plurality of elements and adapted to adjust the orientation of the plurality of elements with respect to the reference plane may be utilized. The local attitude monitoring device may be an encoder, shaft encoder, or gyro. The processing device may include a filter. The plurality of elements may be mounted on a common platform, with the orientation of the plurality of elements with respect to a reference plane being adjusted by adjusting the orientation of the common platform or by adjusting the orientation of the individual elements. The coupled elements may include a substantially linear array or a substantially two-dimensional array.
According to another aspect of the invention, a method of determining a satellite antenna attitude comprises: providing a plurality of coupled antenna elements having an attitude, the antenna elements receiving radio signals from a satellite, calculating an antenna attitude using a phase relationship of the radio signals received by the coupled antenna elements, changing an orientation of at least two of the plurality of coupled antenna elements with respect to a reference plane over a predetermined angle in a synchronized manner, calculating further antenna attitudes using the phase relationship of the radio signals received by the coupled antenna elements as the orientation of at least two of the plurality of coupled antenna elements is changed; and computing an average of the antenna attitudes thus calculated. The changing of the orientation of at least two of the plurality of coupled antenna elements with respect to the reference plane over the predetermined angle may comprise rotating at least two of the plurality of coupled antenna elements about their respective axes of symmetry. The coupled antenna elements may include a substantially linear array or a substantially two-dimensional array.
According to another aspect of the invention, a system for determining a satellite antenna attitude comprises a plurality of coupled antenna elements having an attitude and receiving radio signals from a satellite, a receiver which receives antenna signals from the plurality of coupled antenna elements and determines a phase relationship between the radio signals received by at least two of the plurality of coupled antenna elements, and a processing device which receives from the receiver a signal corresponding to the phase relationship and calculates an antenna attitude therefrom, wherein an orientation of at least two of the plurality of coupled antenna elements with respect to a position of the satellite is changed over a predetermined angle in a synchronized manner and an attitude of the plurality of coupled antenna elements is determined from an average of the antenna attitudes calculated while the orientation is changed. The change in the orientation of at least two of the plurality of coupled antenna elements with respect to the position of the satellite may comprise rotating at least two of the plurality of coupled antenna elements about their respective axes of symmetry. The coupled antenna elements may include a substantially linear array, or a substantially two-dimensional array.
Further features and advantages of the present invention will be apparent from the following desrcription of preferred embodiments and from the claims.